Rectangular ducts used for conducting air as for heating and air conditioning are conventionally made up of relatively short sections. In the most common construction, two workpieces are used for each section, formed longitudinally to L-shape, and having their longitudinal edges seamed, as with a Pittsburgh lock joint. Transverse frames are provided at the ends of the duct sections, so that abutting duct sections may be bolted together at their corners, with the frames held by clips or drive cleats. If one of the sides of the duct section is relatively long, considering the gage of sheet metal utilized, reinforcing bars may be added within the flanges.
Ordinarily such transverse frames are not integral. To join them to the rectangular sheet duct pieces is fairly complex; and extensive use of sealants may be required to assure against leakage of air.
Some duct systems heretofore used have employed transverse frames which are essentially integral, by bending (ordinarily by rolling) transverse flanges whose webs extend outwardly from the duct surfaces at 90.degree. and terminate in outer flanges bent back 90.degree.. Corner pieces are installed to make these rolled flanges into frames; the corner angles of adjacent frames so formed may then be bolted together, and the flanges which project from the adjacent webs may be held together by clips or drive cleats. Such integral transverse flanges may not hold the corner pieces with rigidity and security adequate for the loads likely to be imposed. There is also the problem of leakage of air through the seams immediately adjacent to the integral flanges, because of the inconsistency between longitudinal roll-forming and transverse roll-forming. A typical sheet metal seam, such as a Pittsburgh lock, cannot feasibly be formed immediately adjacent to a transverse bend.
One construction utilizing integral transverse flanges, produced and marketed by Iowa Precision Industries, Inc., adds some rigidity by grooving the sheet metal duct sides inward adjacent to the outwardly-formed webs; however, such inward grooves cut down the duct area through which air may flow. That construction also uses projections on the corner piece ends to hold the ends of flat reinforcing bars, whose inner edges are seated in such grooves and whose outer edges are fitted beneath the 90.degree. bent outer flanges.